The Southeastern Arizona Planning Area is generally characterized by alluvial basins with relatively large reserves of groundwater in gently sloping valleys separated by mountain ranges. Anderson, Freethy and Tucci (1992) divided the alluvial basins of south-central Arizona into five groups based on similar hydrologic and geologic characteristics. One of these, the “Southeast Basins”, covers most of the planning area.

The principal water-bearing deposits in southeast basins are moderately thick sediments deposited prior to the formation of the Basin and Range structure and an overlying layer of lower basin fill that can reach over 1,000 feet thick, derived from the subsequent partial erosion of the ranges (see Figure 3.0-4). Lower basin fill sediments are composed of fine-grained to moderately fine-grained materials. Upper basin fill deposits average about 300 feet thick and are generally composed of sands, gravels, silts, clays and some limestones. Aquifers in this region often consist of two or more water-bearing units separated by a fine-grained unit that forms a leaky confining layer over the lower basin fill. Thin layers of sand and gravel along major streams make up the stream alluvium.

Groundwater generally flows from the margins to the central axis of the basin where most groundwater discharge occurs. Confined groundwater (artesian conditions) can occur within the lower basin fill. Artesian conditions occur in a number of locations in the planning area including: the vicinity of Artesia south of Safford; washes and terraces at the base of the Pinaleño Mountains; the vicinity of Saint David; the San Bernardino Valley Basin; and the Lower San Pedro Basin.

The major groundwater inflow components are mountain front recharge and stream infiltration with some underflow from adjacent up-gradient basins. Outflow consists of evapotranspiration, pumpage, discharge to streams as baseflow and some underflow to down-gradient basins, including into Mexico.

Artesian well in the San Bernardino Valley Basin.

Artesian conditions also occur in the vicinity of Artesia

south of Safford, washes and terraces at the base of the Pinaleño Mountains and in the Lower San Pedro Basin.

The fourteen groundwater basins in the Southeastern Arizona Planning Area are discussed here in four groups according to their general location and similar hydrologic characteristics:

Groundwater basins located in the north and northeastern portion of the planning area are Bonita Creek, Dripping Springs Wash, Duncan Valley, Morenci and Safford. The Safford Basin aquifers are primarily stream alluvium and basin fill, while the other basins also contain aquifers composed of volcanic rock or sedimentary rock (Gila Formation). Groundwater flow is toward the Gila River drainage and the Bonita Creek, Duncan Valley and Morenci basins contribute underflow to the Safford Basin.

Bonita Creek Basin

The portion of the Bonita Creek Basin located within the San Carlos Indian Reservation is characterized by a broad valley bordered by the Nantac Rim and the Gila Mountains. The valley consists of basin fill material with volcanic intrusions where most wells are drilled. The lower part of the basin is characterized by volcanic flows, agglomerates and tuffs interbedded with small sedimentary lenses. In this part of the basin, alluvial deposits along the creek are the main aquifer. Groundwater flow is toward the southeast. Groundwater recharge has been estimated at 9,000 AFA and groundwater in storage estimates vary from 1 to 2 million acre-feet (maf). The reported median well yield from 14 wells is over 1,100 gpm. (Table 3.2-4). Water levels are relatively shallow in the few wells measured in the basin, and all are located near the southern boundary. Water quality data are lacking. The City of Safford operates an infiltration galley along Bonita Creek and conveys water to Safford for municipal use.

Dripping Springs Wash Basin.

Dripping Springs Wash Basin

Dripping Springs Wash is a mountainous basin containing small sediment-filled valleys with relatively little groundwater in storage. The largest valley is north of the Gila River and drained by Dripping Springs Wash. Water producing units consist of younger alluvium and the Gila Conglomerate, with the younger alluvium along Dripping Springs Wash and its tributaries the major water producer. These deposits are reportedly less than 150 feet thick. Consolidated rocks compose the surrounding mountains and contain minor amounts of groundwater. Groundwater flow is towards the Gila River which bisects the basin (Figure 3.3-6). Groundwater recharge has been estimated at 3,000 to 9,000 AFA and groundwater in storage at less than 1maf. Well yields vary widely with a median well yield of about 394 gpm reported (Table 3.6-6). Recent water quality data are lacking.

Duncan Valley Basin

The Duncan Valley Basin consists of an elongate valley filled with sediments, drained by the Gila River and surrounded by low permeability rocks. Younger alluvial deposits along the Gila River and its tributaries are the principle source of groundwater. These deposits are up to 170 feet thick in some locations. Wells also tap the underlying Gila Formation composed of poorly consolidated sand, silt and gravel. The older basin fill contains only minor amounts of groundwater. Groundwater flow is toward the north and west along the Gila River drainage. Groundwater recharge estimates range from 6,000 to 14,200 AFA and groundwater storage estimates range from 9 to 19 maf. The median well yield reported for 165 large diameter wells was 850 gpm (Table 3.7-6). Water levels in measured wells varies from 24 feet to over 500 feet below land surface (bls), with slight water level declines observed from 1990-1991 to 2003-2004 (Figure 3.7-6). Arsenic and fluoride concentrations exceeding drinking water standards have been measured at a number of wells in this basin and a 15-mile reach of the Gila River is impaired due to elevated selenium concentrations (Table 3.7-7).

San Francisco River at Clifton. In the Morenci

Basin groundwater is found primarily in alluvial deposits

along major water courses

Morenci Basin

The Morenci Basin is characterized by steep canyons, mesas and mountains with numerous streams and washes. The basin consists mainly of volcanic rocks (rhyolite and agglomerates overlain by basalt flows). Groundwater is found primarily in alluvial deposits along major water courses and groundwater flow is to the south along the San Francisco River drainage. Groundwater recharge has been estimated at 15,000 AFA and groundwater in storage at 3 maf. Water level change data in the Morenci Basin are available only for the area near Alpine where the measured depth to water is less than 80 feet bls and water levels rose over 15 feet in one well from 1990-1991 to 2003-2004 (Figure 3.9-7). Water quality data shows metal contamination in the vicinity of the Morenci Mine.

Safford Basin

The Safford Basin is a relatively large, alluvial filled depression rimmed by elongated mountain ranges. Basin fill is the major aquifer in all three sub-basins of the Safford Basin. Depth to water is relatively shallow in wells measured near the Gila River, while water levels are generally deeper in wells in the San Simon Valley sub-basin, the southernmost sub-basin. Water levels declined in most wells in the basin that were measured in 1990-1991 and 2003-2004, with the most significant declines south of San Simon where water levels declined by more than 30 feet during this time period (Figure 3.10-6). Water levels exceed 600 feet bls at two wells along the western boundary of the San Carlos Valley sub-basin, the northernmost sub-basin. In one of these wells, water levels declined over 60 feet between 1990 and 2004 (Figure 3.10-7). Fluoride and arsenic concentrations consistently exceed drinking water standards throughout the basin. Most of the groundwater development in the Safford Basin is in the Gila Valley sub-basin, the central sub-basin, which contain the basin’s major population and agricultural centers.

In the San Simon Valley sub-basin a clay deposit, known as the Blue Clay unit, separates the upper and lower aquifers and may be as much as 600 feet thick. Groundwater is found under artesian conditions in the lower aquifer and is generally unconfined in the upper aquifer. Groundwater flow in the sub-basin is toward the north along the San Simon River drainage but also flows toward agricultural pumping centers. The upper aquifer generally contains elevated total dissolved solids (TDS) and fluoride concentrations. The principal aquifer in the Gila Valley sub-basin, located in the middle part of the Safford Basin, is the upper basin fill, underlain by the Blue Clay unit. Groundwater is also utilized from the lower basin fill, which generally is found under artesian conditions and where well discharges may be quite high. Groundwater flow is from south to north along the Gila River drainage. Groundwater in both the upper and lower basin fill may be high in TDS in this sub-basin. The main water-bearing unit in the San Carlos Valley sub-basin, located in the northern part of the Safford Basin, is the upper basin fill, which is found under unconfined conditions. As with the other sub-basins, groundwater in the lower basin fill is generally found under artesian conditions. Groundwater flow in the sub-basin is toward the Gila River drainage.

For more information on Groundwater in individual basins in the Southeastern Arizona Planning Area see the menu to the right.